Method of forming carbon-bonded silicon carbide bodies



United States Patent 3,442,989 METHOD OF FORMING CARBON-BONDED SILICONCARBIDE BODIES Richard D. Hildebrandt, Cupertino, Calif., assignor toKaiser Aluminum & Chemical Corporation, Oakland,

Calif., a corporation of Delaware No Drawing. Filed Mar. 7, 1966, Ser.No. 532,081

Int. Cl. C011) 31/36 US. Cl. 264-29 7 Claims ABSTRACT OF THE DISCLOSUREThis disclosure relates to a method for fabricating silicon carbidebodies and an improved silicon carbide body particularly suitable as asilicon carbide side wall for an electrolytic reduction cell.

This invention relates to a method for fabricating silicon carbidebodies and an improved silicon carbide body.

More particularly, this invention relates to a method of fabricatingsilicon carbide sidewalls for electrolytic reduction cells and toimprove silicon carbide refractories for electrolytic reduction cellsidewalls.

In the production of aluminum by the conventional electrolytic process,the electrolytic cell comprises in general a steel shell having disposedtherein a refractory lining separated from the shell by a layer ofinsulating material. The bottom of the refractory lining along with alayer of electrolytically produced molten aluminum which collectsthereon during operation serves as the cathode. One or more consumablecarbon electrodes is disposed from the top of the cell and is immersedat its lowest extremity into a layer of molten electrolyte which isdisposed in the cell. In operation. the electrolyte or bath which is amixture of alumina and cryolite is charged to the cell, and an electriccurrent is passed through the cell, from the anodes to the cathode viathe layer of molten electrolyte while oxygen collects at the anode. Acrust of solidified electrolyte and alumina forms on the surface of thebath, and this is usually covered over with additional alumina.

Traditionally the refractory lining for the reduction cell has been ofcarbon, either carbonaceous material rammed into place or preformedcarbon blocks. Carbonaceous materials that have been used includepetroleum coke, anthracite, gilsonite coke, or various mixtures thereof.It has long been thought desirable to form the sidewalls of a differentmaterial than the bottom cathode area of the cell lining. If this weredone and a poor electrical conductor selected as the sidewall material,the flow of current from the anode to the cathode floor of the cell viathe molten metal pad could be more easily controlled and there would beless probem with stray current fiow through the sidewall. A materialthat was early looked at for this was silicon carbide. An additionaladvantage that can be realized from the use of silicon carbiderefractory brick to replace thick carbon cell sidewalls is that the cellcavity capacity is increased and consequently the cell productivity isincreased also. However, if the standard silicon carbide refractoriesthat are bonded with silicon nitride or silicon oxynitride are used, thedisadvantage of erosion by molten fluoride electrolyte and contaminationof the cell aluminum product with silicon has resulted.

Thus, in general the properties of an ideal sidewall material forelectrolytic reduction cells would be poor electrical conductivity, highdensity or low porosity, high strength, and good thermal conductivity,or at least a thermal conductivity appropriate to the cell design, sothat frost heave isotherms are minimized in the sidewall area. Frostheave isotherms refers to the penetration of the sidewall by and thefusing therein of the molten salts present in the reduction cell bath.The action is similar to the frost heave that may occur in thefoundations of cold storage buildings or cause the lifting of thesurface of the ground with certain types of soils in frosty weather,where the growth of columnar ice crystals at the freezing interface candevelop large forces.

A method has now been developed for producing a silicon carbide bodywhich body is suitable for use in electrolytic reduction cell sidewallsand meets these desired properties. Specifically, there has beendeveloped a process and a unique carbon bonded silicon carbide bodywhich has the high compressive strength and good electrical resistivityand thermal conductivity of conventional silicon nitride or siliconoxynitride bonded silicon carbide refractories and also has the higherosion resistance of the more conventional carbon bonded carbonrefractory. More specifically, there has been developed a method offabricating a silicon carbide body comprising preparing a mixture ofsilicon carbide particles, from 4%12% of a cokable, carbonaceous bondingagent, and from 0.5 70-10% added elemental sulfur. These percentagesareby weight of total mixture. Any suitable cokable carbonaceous bondingagent may be used such as tar, asphalt or pitch. Applicant has foundthat pitch is particularly suitable. Pitches having softening pointsfrom 55 C. up to and including 170 C. have been successfully used. Thesoftening point of the pitch materials is determined by the cube inwater method which carries the American Society for Testing Materialsdesignation D61-38. In general, this method consists of measuring thetemperature at which a /2 cube of pitch when suspended at a specifieddistance above the bottom of a container will elongate and touch thebottom of the container.

The materials may be all mixed together initially according to thisinvention, but for best results they should be added incrementally. Themixture is heated to a temperature at which the bonding agent softensand flows to coat the particles and form a uniform mixture. The uniformmixture is then compressed either by ramming, tamping or pressing toform a body having a density of at least 2.5 grams/cc. It is essentialthat the green mixture achieve atleast this density in order that aproduct having a satisfactory baked density may be attained. Thecompressed body is then baked at a controlled rate of heating to permitslow evolution of vapors whereby a dense baked product is formed.

It is desirable that the silicon carbide particles be size graded fordense packing in order that the density criterion can be satisfied. Anysuitable size gradation which will achieve dense packing can be used.Applicant has found that a particularly suitable size gradation for thesilicon carbide particles comprises 1%%10% of a 4 and +8 mesh Tylerscreen fraction, 17%10% of a -8 and +14 mesh Tyler screen fraction,22%-10% of a 14 and +28 mesh Tyler screen fraction, 8 /2 %10% of a -28and +48 mesh Tyler screen fraction, 18%-10% of a +48 and mesh Tylerscreen fraction, 8%10% of a 100 and +200 mesh Tyler screen fraction,10%20% of a 200 and +325 mesh Tyler screen fraction, and 12%%20% of a325 mesh Tyler screen fraction. These percentages are by weight ofsilicon carbide present.

As has been indicated, a highly satisfactory cokable carbonaceousbonding agent is pitch, particularly pitch having a cube in watersoftening point of about 55 C. Under these conditions, a suitabletemperature at which the mixture should be heated in order for thebonding agent to soften and flow to coat the particles and form auniform mixture is about C.

The baking is desirably carried out in a reducing atmosphere. The bakingmust be at a controlled rate of heating to permit slow evolution ofvapors. It is believed that the sulfur present in the mixture chemicallystrips the hydrogen content of the pitch as H 8 and promotes an earlycondensation or concentration of the carbon content of the pitch into awell ordered high carbon resin. Thus, the vapors that are evolved are H8, sulfur, and other vaporous products of the baking out stage of theprocess. It has been found that a controlled rate of not more than 50 C.temperature rise per hour to a temperature from 700 C.-l500 C. permitsthis slow evolution of vapors to occur and results in a dense bakedproduct being formed. The most desirable temperature range to which thebody should be baked is from 900 C.1200 C. Obviously, any suitabletemperature and length of holding time within this range can beutilized. For example, if one desires to bake only to 900 C., asatisfactory product can be achieved by holding the body at thistemperature longer than if one desires to use a final baking temperatureof 1200 C. where a shorter holding time can be utilized.

Although as has been indicated above, all of the mapoint of the sulfuryields an increased pitch-coke binder residue over that of normal pitch.Hence, with a pitchsulfur binder, after compaction and baking, asignificantly more dense and strong carbon bonded silicon carbide bodyresults. As has been stated, such a bonded body lends itself to eitherthe making of refractory-type brick or slabs of larger rammed monolythicunbaked shaped articles.

Table I shows the physical properties of various baked silicon carbidebodies including a silicon carbide body prepared in accordance with thisinvention. The molten fluoride spin erosion test results shown in TableI clearly indicate the superior erosion resistance of the siliconcarbide bodies produced according to this invention over the commercialsilicon carbide brick. The molten fluoride spin erosion test involvessuspending a wafer of the material to be tested in a bath of molten(1000 C.) cryolite containing 8% calcium fluoride and 6% A1 0 The wafersare from inch thick, 4 inch wide and 2 /2 inches long. The wafers areagitated in the bath by rotation at approximately 200 revolutions perminute and the volume loss is periodically determined.

TABLE I.PHYSICAL PROPERTIES OF VARIOUS BAKED SILICON CARBIDE BODIESGreen Baked Elec. Comp. Percent bake change Molten fluoride Compositiontype density, density, res., strength, spin erosion test g./cc. g./cc.n-cm. p.s.i. Ave. lineal Wt. loss (1,000 0.)

Commercial SiG brick silicon oxynitride 2. 5-2. 6 6, 500 From 510%volume erosion bun e after 5 hours.

Carbon bonded SiC (Sm-{% C. pitch 2. 2. 63 0.2 6, 400 0.0 1. 55 Only 51.surface erosion after 5 +2% sulfur hrs. 5% volume.

Carbon blofnd)ed SiO (Sic-140% 55 C. pitch 2, 58 2. 37 2, 000 +3. 1. 75Not tested.

no su ur As above (SiC +5% 150 C. pitch-l-no sulfur) 2. 49 2. 46 41 90006 1 Do,

Carbon bonded carbon Unaffected by erosion testing terials may beinitially mixed together and a satisfactory product formed, it has beenfound that a particular sequence of steps in the method would producethe most satisfactory results. This method for fabricating the siliconcarbide bodies comprises uniformly heating a mixture of silicon carbideparticles that have been size graded for dense packing to about C. C.From 4%- 12% of pitch binder is then added to the aggregate with mixing.Mixing continues while the mixture is heated to about C. From 0.5%10%elemental sulfur in the form of flowers of sulfur is then added to themixture and the heating and mixing is continued until a temperature ofabout C. is reached so that the bonding agent flows to coat theparticles whereby a uniform mixture is attained. The mixture is thencompressed in any suitable manner, e.g., by tamping into place in thecell, to form a body having a density of at least 2.5 grams/cc. The bodyis then baked at a controlled rate of not more than 50 C. temperaturerise per hour to a temperature from 700 C.l500 C.

The silicon carbide containing composition resulting from the practiceof this invention may be ram-med in place in the cell and baked outtherein as indicated above or it may be compressed into a preformedproduct which is baked out before being placed into the cell. It hasbeen found that for best results, the quantity of cokable carbonaceousbonding agent should be about 5% and the quantity of elemental sulfurabout 2% when the desired size gradations of silicon carbide particlesare used.

The sulfur additive serves several purposes in the mixture. Sulfur isthoroughly miscible with coal tar pitch and makes it much more fluid atnormal mixing temperatures, that is, from 80 C.l 50 C. This permitssuperior wetting and compaction with a silicon carbide aggregate. Sulfuris well known as a dehydrogenization agent. Hence, during baking ofpitch sulfur bonded silicon carbide bodies, the sulfur chemically stripsthe hydrogen content of the pitch as H 8 and promotes an earlycondensation or concentration of the carbon content of the pitch into awell ordered carbon resin. This resin despite the total loss ofremaining sulfur by heating to above the boiling As shown in Table 1,commercial silicon carbide brick, that is silicon carbide brick and thatis silicon oxynitride bonded, has a baked density of from 2.5-2.6grams/cc. It has a high electrical resistivity and a high compressivestrength of 6500 pounds per square inch. However, this brick suifersfrom a 5 %-10% volume erosion after five hours of the molten fluoridespin erosion test. Carbon bonded silicon carbide brick, that is carbonbonded silicon carbide that has been prepared in accordance with thisinvention by mixing silicon carbide particle size graded for densepacking with 6% of 55 C. pitch and 2% sulfur, as shown in this examplehas a green density of 2.70 grams/cc, a baked density of 2.63 grams/cc.and a satisfactory electrical resistivity. The compressive strength wascomparable to that of a commerial silicon carbide brick being about 6400pounds per square inch. In addition thereto, and surprisingly, the bodyprepared according to the invention showed only slight surface erosionafter five hours in the molten fluoride spin test. The amount of erosionwas less than 5% by volume. Where sulfur is not used in the preparationof the carbon bonded silicon carbide, the bodies have unsatisfactorycompressive strength as shown in Table I. Table I includes for purposesof comparison carbon bonded which as indicated was uneffected by theerosion testing.

Throughout an extensive series of tests, green densities :of greaterthan 2.5 grams/ cc. have been consistently achieved in silicon carbidebodies made according to this invention. All of these bodies showed thesuperior properties characteristic of the process of this invention.Thermal conductivity measurements were also made between the siliconcarbide bodies prepared according to this invention and commercialsilicon carbide brick by placing the ends of 4.5 inch thick samples in980 C. molten bath and observing the time required to reach severaltemperature levels at the other radiated shield end. Both bodies reached60 C. in 30 seconds. The carbon bonded silicon carbide preparedaccording to this invention reached 306 C. in 4 minutes and thecommercial silicon carbide brick reached 306 C. in 3.95 minutes. Thecarbon bonded silicon carbide prepared in accordance with this inventionreached 523 C. in minutes and the commercial silicon carbide brickreached 534 C. in 10 minutes.

It is to be understood that many changes and variations can be made tothe above outlined process and product description without departingfrom the spirit and scope of the instant invention. As has beenindicated, cokable carbonaceous bonding materials of various types maybe used and specifically pitches having softening points up to 170 C.have been successfully used. The mixture can also include carbon orgraphite aggregate, furfural or furfural alcohol impregnation of thebaked body and variations in pitch and sulfur levels as has beenindicated without departing from the spirit and scope of the instantinvention.

What is claimed is:

1. A method of fabricating a silicon carbide body comprising:

(a) preparing a mixture of silicon carbide particles,

from 4% to 12% of a cokable, carbonaceous bonding agent, and from 0.5%to 10% added elemental sulfur, said percentages being by weight of totalmixture;

(b) heating the mixture to a temperature from about 80 C. to about 150C. at which the bonding agent softens and fiows to coat the particlesand form a uniform mixture;

(c) compressing the uniform mixture to density to a density of at least2.5 grams/cc.;

(d) baking the compressed body at a controlled rate of heating to permitslow evolution of vapors to a temperature not greater than about 1500 C.whereby a dense baked product is formed.

2. The method of claim 1 wherein the silicon carbide particles are sizegraded for dense packing.

3. The method of claim 1 wherein the cokable, carbonaceous bonding agentis pitch.

4. The method of claim 1 wherein the mixture is heated to about 150 C.

5. The method of claim 1 wherein the baking is at a controlled rate ofnot more than C. temperature rise per hour to a temperature from 700 C.to 1500 C.

6. The method of claim 1 wherein the baking is at a controlled rate ofnot more than 50 C. temperature rise per hour to a temperature from 900to 1200 C.

7. A method of fabricating a silicon carbide body comprising:

(a) uniformly heating a mixture of silicon carbide particles size gradedfor dense packing to about C. to C.;

(b) adding about 4% to 12% pitch binder to the aggregate with mixing;

(c) heating, while mixing continues, the mixture to about C.;

(d) adding 0.5% to 10% elemental sulfur to the mixture;

(e) continuing the heating and mixing to about C. so that the bondingagent flows to coat the particles whereby a uniform mixture is attained;

(f) compressing the mixture to form a body having a density of at least2.5 grams/co;

(g) baking the body at a controlled rate of not more than 50 C.temperature rise per hour to a temperature from 700 C. to 1500 C.

References Cited UNITED STATES PATENTS 2,131,021 9/1938 Bemis 264292,637,072 5/ 1953 Greaves 26429 2,799,912 7/1957 Greger 26429 2,807,85610/ 1957 Frosch 264109 3,092,437 6/1963 Carter et al 26429 3,166,614 1/1965 Taylor 26429 3,168,602 2/1965 Davies et a1 26429 DONALD J. ARNOLD,Primary Examiner.

U.S. Cl. X.R.

